1. Field of the invention
The present invention relates generally to a suspension control system for an automotive vehicle for effectively adjusting suspension characteristics so as to obtain better shock absorbing ability. More specifically, the invention relates to a semi-action control system for an automotive vehicle which reduces frequency of switching of suspension characteristics for longer life of components associated for adjustment of suspension characteristics.
2. Description of the Background Art
Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 63-112914 discloses a suspension control system for an automotive vehicle. In the disclosed suspension control system, relative displacement between a vehicular body as a sprung mass and a suspension member rotatably supporting a vehicular wheel and thus serving as an unsprung mass is monitored. When the direction of variation of relative distance is to increase a distance from a neutral position, then, the suspension characteristics are assumed for a softer suspension and to decrease a distance from the neutral position, the suspension characteristics are assumed for a harder suspension. The suspension control system detects input vibration frequency so as to assume softer suspension characteristics when the vibration frequency requires switching of suspension characteristics at a higher frequency than an actual switching frequency of suspension characteristics due to mechanical lag.
In such suspension control system, harder suspension characteristics are assumed both for bounding and rebounding stroke of vibration when harder suspension characteristics are selected, and softer suspension characteristics are assumed both for bounding and rebounding strokes when softer suspension characteristics are selected. Such manner of suspension control encounters drawbacks in certain vehicular traveling conditions. Namely, when vibration a composite vibration of high frequency components due to small undulation of the road surface and low frequency components due to vehicular body attitude change, difficulty may be encountered to select the suspension characteristics. Namely, if harder suspension characteristics are selected in response to the composite vibration, the suspension characteristics will become too stiff to effectively absorb road shock. On the other hand, if softer suspension characteristics are selected, suspension characteristics will become toosoft for suppressing vehicular body attitude change to cause degradation of vehicular driving stability.
Furthermore, at bouncing in which damping force generated by the suspension system alternatively serves for amplifying and absorbing vibration energy, the suspension. characteristics should be switched between soft and hard characteristics repeatedly. This causes high frequency of necessity of switching of the damping characteristics. This clearly shortens the life of the mechanism for switching the damping characteristics.